Frequently, large elongated composite structures of fibre-reinforced polymer are manufactured as shell parts in moulds where a first side and a second side of the structure are manufactured separately and assembled afterwards. Thus, wind turbine blades are usually manufactured as shell parts in moulds, where the pressure side and the suction side, respectively, are manufactured separately. Afterwards, the two blade halves are glued together, often by means of internal flange parts.
Large composite structures may be manufactured in various ways. Vacuum infusion or VARTM (Vacuum Assisted Resin Transfer Moulding) is one method, which is typically employed for manufacturing composite structures such as wind turbine blades comprising fibre-reinforced matrix material. During the manufacturing process, liquid polymer, also called resin, is filled into a mould cavity, in which fibre material, also called fibre lay-up, has been previously inserted and where vacuum is generated in the mould cavity hereby drawing in the polymer. The polymer can be thermoset plastic or thermoplastic. Typically, uniformly distributed fibres are layered in a first rigid mould part, the fibres being rovings, i.e. bundles of fibre bands, bands of rovings or mats, which are either felt mats made of individual fibres or woven mats made of fibre rovings. Subsequently, a second mould part, which is often made of a resilient and flexible polymer foil, also called a vacuum bag, is placed on top of the fibre material and sealed against the first mould part in order to generate a mould cavity. By generating a vacuum, typically 80-95% of the total vacuum in the mould cavity between the first mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained therein. So-called distribution layers or distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases, the polymer applied is polyester or epoxy, and the fibre reinforcement is often based on glass fibres or carbon fibres. However, other types of fibres, such as natural fibres and steel fibres, may also be used.
During the process of filling the mould, a vacuum is generated via vacuum outlets in the mould cavity, said vacuum in this connection being understood as an underpressure or negative pressure, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels, the polymer disperses in all directions in the mould cavity due to the negative pressure as the flow front moves towards the vacuum channels.
Often, the composite structures comprise a core material covered with a fibre-reinforced material such as one or more fibre-reinforced polymer layers. The core material can be used as a spacer between such layers to form a sandwich structure and is typically made of a rigid light-weight material in order to reduce the weight of the composite structure. In order to ensure an efficient distribution of the liquid resin during the impregnation process, the core material may be provided with a resin distribution network, e.g. by providing channels or grooves in the surface of the core material.
Another method for manufacturing composite structures is resin transfer moulding (RTM) which is similar to VARTM. In RTM, the liquid polymer is not drawn into the mould cavity due to a vacuum generated in the mould cavity. Instead the liquid resin is forced into the mould cavity via an overpressure at the inlet side.
A third method for manufacturing composite structures is pre-preg moulding. Pre-preg moulding is a method in which reinforcement fibres are pre-impregnated with a precatalysed resin. Typically, the resin is solid or nearly solid at room temperature. The pre-pregs are arranged by hand or machine onto a mould surface, a vacuum bag, and heated to a temperature where the resin is allowed to reflow and eventually cured. This method has the main advantage that the resin content in the fibre material is accurately set beforehand. The pre-pregs are easy and clean to work with and make automation and labour saving visible. The disadvantage with pre-pregs is that the material costs are higher than for non-impregnated fibres. Further, the core material needs to be made of a material which is able to withstand the process temperatures needed for bringing the resin to reflow. Pre-preg moulding may be used both in connection with an RTM and a VARTM process.
Further, it is possible to manufacture hollow composite structures in one piece by use of outer mould parts and a mould core. Such a method is e.g. described in EP 1 310 351 and may readily be combined with RTM, VARTM and pre-preg moulding.
Certain composite structures, such as wind turbine blades, have become increasingly longer over the years, and today blades of more than 60 m are manufactured. As the production facilities for large composite structures, such as wind turbine blades, are usually not located next to the site of use of the structures, the structures need to be transported from the production site to the site of use. Transportation of such large structures is often problematic as they are usually transported by road at least part of their way from the production facility to the site of use. Therefore, there is a need for blades that may be transported more easily.
Therefore, it has been proposed to separate wind turbine blades into two or more separate blade sections and then assemble the blades at the site of the wind turbine plant. Thereby, it is possible to manufacture the separate blade sections in smaller moulds and it is less problematic to transport the blade sections than a blade. An example of such blade is described in EP 1 584 817 A1. However, producing the separate blade sections in separate moulds may create problems in obtaining a perfect fit between the blade sections and thereby in assembling the blade sections into a wind turbine blade.